7. Analytical Techniques I

Question 1

What would happen to a molecule when put through a mass spectrometer?

Answer 1

the molecule will break up and give a series of peaks

Question 2

What are the peaks on a mass spectrometer caused by?

Answer 2

by the broken up fragments

Question 3

What will the peak with the largest m/z be due to? what will it equal?

Answer 3

. the complete molecule
. equal to the Mr of the molecule

Question 4

What is the peak with largest m/z called?

Answer 4

The parent ion OR the molecular ion

Question 5

What is the molecular ion for butane?

Answer 5

C4H10 +

Question 6

When organic molecules are passed through a mass spectrometer, what two things does it detect?

Answer 6

. the whole molecule
. fragments of the molecule

Question 7

What do several peaks in the mass spectrum occur from?

Answer 7

fragmentation

Question 8

Why does the molecular ion fragment?

Answer 8

due to covalent bonds breaking

Question 9

Write an equation showing the molecular ion formed from fragmentation using M.

Answer 9

M ——> [M]+ + e-

Question 10

In the equation, M—-> [M]+ + e-, what do the [M]+ and the e- stand for?

Answer 10

[M]+ = the ion responsible for the peak
e- = a free radical

Question 11

When is there a greater peak intensity?

Answer 11

when there’s a more stable ion

Question 12

What are some examples of stable ions?

Answer 12

. carbocations R+ (e.g. CH3CH2+)
. acylium ions [R-C=O]+

Question 13

What are acylium ions?

Answer 13

cations (positive ion) with the formula RCO+

Question 14

What are carbocations?

Answer 14

ions in which a carbon atom carries a positive charge

Question 15

How do carbocations occur?

Answer 15

. when a carbon atom only has 3 bonds instead of the usual 4
. results in an electron deficiency

Question 16

Is the molecular ion fragmented?

Answer 16

NO.

Question 17

Why is the mass/charge ratio equal to the Mr?

Answer 17

because the charge of the molecular ion is 1+

Question 18

Write the equation for the formation of the molecular ion of propanal.

Answer 18

CH3CH2CHO —–> [CH3CH2CHO]+ + e-

Question 19

What can fragmentation patterns also distinguish between with 2 compounds? (e.g. propanal and propanone)

Answer 19

structural isomers

Question 20

What does infrared spectroscopy provide information about?

Answer 20

the types of bonds present in a molecule

Question 21

Certain bonds in a molecule absorb infra-red radiation at characteristic frequencies which causes….?

Answer 21

the covalent bonds to vibrate

Question 22

At what wavelength can functional groups be identified?

Answer 22

ABOVE 1500 cm-1

Question 23

What is the area below 1500 cm-1 known as?

Answer 23

fingerprinting

Question 24

Why don’t we use the fingerprinting area not be used to identify functional groups?

Answer 24

because it’s complicated and contains many signals. Difficult to pick out functional group signals.

Question 25

Why is the area below 1500 cm-1 known as the fingerprint?

Answer 25

because it's unique for every compound (like a fingerprint is to every person)

Question 26

Can molecules that change their polarity as they vibrate absorb infrared radiation?

Answer 26

YES

Question 27

Why don't molecules such as H2, O2 and N2 register on an infrared spectrum?

Answer 27

. because they cannot change their polarity as they vibrate . they can absorb infrared but just don't register

Question 28

What is mass spectrometry used to identify?

Answer 28

different isotopes and overall relative atomic mass of an element

Question 29

How can the relative atomic mass be calculated?

Answer 29

Ar = m/z x abundance / Total abundance